Potential candidates for City Councilmember Alan Gerson’s seat now find themselves running against the man they had intended to replace.

Gerson’s vote more than a month ago to give Mayor Mike Bloomberg a chance at another term also gave Gerson himself the opportunity to run for re-election in 2009 after his two terms are up. Gerson plans to take advantage of this chance for a third term, which means that those who expected to run for an open seat are now likely facing an incumbent.

The First Council District includes Lower Manhattan, Soho, Washington Square, the South Village, the Lower East Side, Little Italy and part of Chinatown.

Margaret Chin and Pete Gleason, the only two candidates who officially joined the race before Gerson added his name to the list, still plan to run and are trying not to let the switch-up affect their campaigns.

“I’m going to do what I was doing anyway,” said Chin, a Hanover Square resident who left her job at Asian Americans for Equality to campaign full time. “I just have to work a little harder.”

Gleason, a lawyer and former firefighter who lives in Tribeca, spent much of the last year criticizing Julie Menin, the Community Board 1 chairperson, whom Gleason presumed to be his top opponent. But after Gerson joined the race, Menin said she would not run, and Gleason quickly made the transition to campaigning against Gerson instead.

“Both of them are formidable opponents,” Gleason said. He now has to battle the power of Gerson’s incumbency, rather than Menin’s wealth, he said.

Menin had been an early favorite in the race, though she never formerly joined it. Now that Gerson is running, she said she looks forward to staying chairperson of the community board and spending time with her 3-year-old twins and 5-year-old son.

“I’m O.K. waiting and possibly running in five years,” Menin said.

She did not say whether she would re-enter the race if Gerson backed out, saying chances to win an open seat do not come often.

Under Community Board 1’s term limit rules, Menin could potentially stay chairperson of the board until 2012.

Another prospective candidate and Gerson supporter, Jan Lee, who owns a furniture store in Chinatown, said he still has not made a decision on whether to run, though he said about a year ago that he was months away from deciding.

“This has really thrown a big wrench into it,” Lee said of Gerson’s decision.

Borough President Scott Stringer had considered a run for public advocate but he recently announced that he would run for re-election instead. Prior to the announcement, Gerson said if Stringer chose not to run, he would “possibly” run for borough president himself, which would take him back out of the City Council race and open it up again.

Two more candidates who would make a splash if they jumped into the race are Madelyn Wils, who works for the city’s Economic Development Corporation, and Kathryn Freed, a judge who held Gerson’s seat before he was elected in 2001. Either one would be a strong candidate, several people said. Gleason even said he would drop out if Freed ran.

Freed said that she has pretty much decided not to run. While she is not enamored of her job as a judge, financially she could not afford to sacrifice her salary while running for the council, she said.

“It would be fun,” she said wistfully. “I miss being in the City Council.”

Freed opposes term limits and said that if she hadn’t been forced to leave the council at the end of her second term in 2001, she would likely still be there.

Wils, who chaired C.B. 1 and later joined E.D.C. as vice president for planning and development, when recently asked, would not rule out the possibility of running against Gerson.

“My intention right now is to stay where I am,” she said. “Right now, all I’m thinking about is the position I hold. I’m very busy.”

John Fratta, a C.B. 1 member who ran for the First Council District seat in 2001 isn’t running this time, and was backing Menin before she dropped out. Fratta thinks people are so incensed over Gerson’s term limit vote that several more will join the race.

“You’re going to see a few candidates coming out of the woodwork to run,” Fratta said. “It’s going to be fun.”

While people speculate about potential candidates, Chin and Gleason are plowing forward with their campaigns. Gleason said he raised more than $10,000 at his first fundraiser two months ago with promises for $25,000 more, and Chin has raised more than $80,000 so far. Chin recently opened her campaign office, the earliest she has ever opened an office in her several attempts for the City Council.

Gleason, 45, is defining this campaign as a marathon, unlike his unsuccessful 90-day sprint against Gerson in 2003. His first fundraiser gave supporters a chance to hold actor Gary Cooper’s Oscar (his daughter Maria Cooper Janis, a Gleason supporter, brought it along) and Olympic rower Susan Francia’s gold medal from Beijing.

Gleason and Chin are both courting the Downtown Independent Democrats, and Gleason already has the support of Democratic District Leader Jean Grillo. D.I.D. President Sean Sweeney attended Gleason’s fundraiser, but he also attended an open house Chin held several weeks ago.

Adam Silvera, a district leader and D.I.D. club member, said he counts both Chin and Gleason as friends. Silvera said he was disappointed with Gerson’s term limits vote, and he thinks Gleason would be a breath of fresh air who will “say the things that need to be said.” Silvera also admires Chin’s advocacy for affordable housing.

“I’m torn,” he said. “It will be a tough decision.”

Grillo supported Gerson over Gleason in 2003 and worked with both of them to found the Tribeca Community Emergency Response Team, or CERT. But she said she is supporting Gleason now because it’s time for new blood in the council, especially given the delays in rebuilding the World Trade Center. That’s one of Gleason’s favorite issues, and he even created a mock Monopoly-style board game lampooning the bureaucracy surrounding the site.

Gleason criticized Gerson for having difficulty making decisions and not using his position to speed progress at Ground Zero. Gleason calculates that taxpayers will have spent nearly $1 million on Gerson’s salary by the end of his second term, and he asks rhetorically if the constituents have gotten their money’s worth.

Gerson, 51, said the City Council was given very little authority on the World Trade Center rebuilding. However, he said he used what power he had to form the council’s Lower Manhattan Redevelopment Committee and focus on the issues that are important to the community, including progress on the performing arts center, safety at the Deutsche Bank building and funding for a new Fiterman Hall for Borough of Manhattan Community College.

“If Pete Gleason had a better idea and thought we weren’t doing enough, he could have come to a hearing or written a letter,” Gerson said. “He didn’t do any of that.”

Chin, 55, said she was disappointed with Gerson’s vote on term limits, but beyond that she would not criticize him. She said she had several conversations with him about her candidacy before he joined the race, and she had hoped for his endorsement. She has not altered her strategy, but said she will stick to her plan of meeting as many people across Lower Manhattan as she can.

A recent open house at Chin’s new Chatham Green headquarters drew dozens of people. With old friends embracing, a long table holding platters of food and Chin beaming at the center of it all, the campaign event felt more like a family reunion. The crowd, mostly from Chinatown, chattered away in a mix of Cantonese, Toisanese and Mandarin. Born in Hong Kong, Chin speaks all three dialects.

When asked why they support Chin, the attendees invariably started their explanation by saying how long they had known her and how she had helped them.

Steven Wong, 54, said he met Chin 26 years ago, when he first arrived in the United States and didn’t know any English. Chin helped him sign up for language classes, and she has never stopped working for the community, he said.

“She don’t care City Council or not City Council — she’s still doing more than the City Council job,” Wong said. “She doesn’t get the pay, but she does the work.”

Chin has run for this City Council seat three times before, but she said she has a good feeling about this race, despite Gerson’s entry. She has a longer record now on community issues, and more Chinatown residents have become citizens and have registered to vote. Particularly in this presidential election year, Chin said she has seen many people become more politically active. Several people said Gerson has waning support in Chinatown.

“It’s exciting,” said Alan Tung, Chin’s husband, who teaches fifth grade at P.S. 3 in the Village. “Hopefully, this will be the time she wins.”